Electrostatic magnetic developing apparatus

ABSTRACT

An arrangement of spaced magnetic brushes for developing latent electrostatic images wherein the magnetic fields associated with the brushes overlap so that magnetizable carrier particles are carried from one brush to the other across the space therebetween thereby presenting a continuous magnetic blanket of developing material during development.

United States Patent Nielander Feb. 8, 1972 [54] ELECTROSTATIC MAGNETICDEVELOPING APPARATUS 21 Appl. No,: 830,285

[52] US. Cl ..l18/637 [51] Int. Cl ..B05b 5/00 [58] Field of Search..118/637,623,602,636, 603; 117/17.5, 37 L X; 198/41; 209/216, 219

[56] References Cited UNITED STATES PATENTS 3,543,720 12/1970 Drexler etal ..1 18/637 2,745,549 5/1956 Syodig 209/219 3,113,042 12/1963 Hall118/637 3,133,834 5/1964 Sowiak ..1 18/637 3,457,900 7/1969 Drexler..117/l7.5 X 3,472,205 l0/l965 Tsuchiya et al ..1l7/17.5 X

WWW/W 1 /z 57 y/ 1 FOREIGN PATENTS OR APPLICATIONS 17,228 8/1966 Japan..1 18/637 OTHER PUBLICATIONS TBM Technical Disclosure Bulletin. Crosset all. 37 38 Magnetic Brush Developer". Vol. 9, No. 9 (Feb. 1967) pp.

1090.1091-C0pyin 346/74 ESX Primary Examiner-Morris KaplanAttorneyNorman E. Schrader, James J. Ralabate and Bernard A Chiama [5 7]ABSTRACT An arrangement of spaced magnetic brushes for developing latentelectrostatic images wherein the magnetic fields as- 'sociated with thebrushes overlap so that magnetizable carrier particles are carried fromone brush to the other across the space therebetween thereby presentinga continuous magnetic blanket of developing material during development.

7 Claims, 3 Drawing Figures PATENTEU FEE 8 i972 SHEET 1 BF 3 INVENTORWILLIAM B. NIELANDER KM J QA .4 T TORNEY ELECTROSTATIC MAGNETICDEVELOPING APPARATUS This invention relates to a developing apparatus ofthe type employing magnetic fields for forming brush bristles that arecarried into contact with a latent electrostatic image to be developed.

Generally, for magnetic bnrsh development, there is employed a singlemagnetic brush arranged in close spaced relationship relative to asurface carrying a latent electrostatic image. Since the speed ofdevelopment, especially for solid area images is quite limited with theuse of a single brush, there have been attempts to utilize twomagneticbrushes, each having a mechanism for introducing developingmaterial individually. For still higher speeds, and better solid areacoverage, the use of two brushes are not adequate. Utilizing more thantwo brushes, while being effective for high-speed developing, involvesthe need for relatively large spaces to contain development equipmentand complex tonerdispensing equipment.

Therefore, it is a principal object of the present invention to arrangemagnetic brushes so that a maximum development zone is provided for anyparticular amount of developing to be acquired. I

Another object of the invention isto conserve machine space byminimizing or eliminating the need for extra developing material suppliesand accessories for multiple magnetic brush units.

Still another object of the invention is to arrange multiple magneticbrushes so as to allow more rapid activation and deactivation of theunit during its programming sequence.

The foregoing objects are attained by an arrangement which includes atleast two magnetic brushes spaced apart to such an extent that magneticbristles are adapted to be carried from one brush to the succeedingbrush, thereby supplying this brush with material. The resultant flow ofdevelopment material spans the top portions of the two brushes and thespace therebetween with the development material being in contact withan image to be developed throughout this travel. While in this space andduring movement from one brush to another, the development material isin the state of agitation and in good developing condition. More uniformmixture of the ingredients of the material is also available beforebeing applied to the second brush.

For a better understanding of the invention as well as other objects andfurther features thereof, reference is had to the following detaileddescription of the invention to be read in conjunction with theaccompanying drawings'in which:

FIG. 1 is a schematic view of a reproduction machine show ing variouselectrostatic processing components;

FIG. 2 is an enlarged cross-sectional view of one of the developingunits taken along a line parallel to the path of movement of aphotoconductor element; and

FIG. 3 is a cross-sectional view of the developing unit taken along aline normal to the path of movement of the photoconductor.

For a general understanding of the illustrated copier/reproducfionmachine, in which the invention may be incorporated, reference is had toFIG. 1 in which the various components for the machine are schematicallyillustrated. As in all electrostatic systems as well as'a xerographicmachine of the type illustrated, a light image of a document to bereproduced is projected onto the sensitized surface of a xerographicplate to form an electrostatic latent image thereon. Thereafter, thelatent image is developed to fonn a xerog aphic powder image,corresponding to the latent image on the plate surface. The powder imageis then electrostatically transferred to a support surface to which itmay be fixed by a fusing device whereby the powder image is causedpermanently to adhere to the support surface.

In the illustrated machine. an original D to be copied is placed upon atransparent support platen fixedly arranged relative to an illuminationlamp assembly positioned at the upper end of the machine as viewed inFIG. 1. While upon the platen, a programming system for the machineintroduces a lamp control circuit to cause successive energization ofthe lampsiirithe lamp assembly 10 for impinging light rays upon theoriginal thereby producing image rays which when acted upon byseparation filters correspond to the color informational areas on theoriginal. The image rays are projected by means of an optical lenssystem 11 for exposing the photosensitive surface of a xerographic plateat the exposure station A, the plate being in the form of a flexiblephotoconductive belt 12 arranged on a belt assembly generally indicatedby the reference numeral 13.

The .photoconductive belt assembly 13 may be mounted upon the frame ofthe machine and is adapted to drive the selenium belt 12 at a constantrate in the direction of the arrow as shown in FIG. 1. During thismovement of the belt,

the light-imaging rays of an original are successively flashed.

The flash exposure of the belt surface to the light image discharges thephotoconductive layer in the areas struck by' light, whereby thereremains on the belt an electrostatic latent image for each exposure,each being in image configuration corresponding to the light imageprojected from the original D on the supporting platen through thecorresponding separation filter. As the belt surface continues itsmovement, the latent electrostatic images pass through a developingstation B at which there is positioned a developer assembly generallyindicated by the reference numeral 15 and where the belt is maintainedin a flat condition. The developer assembly 15 comprises a plurality ofdeveloping devices 16, l7, l8 and 19 each of which contains a differentcolor-developing material to provide individual development of theelectrostatic images.

The successively developed electrostatic images are transported by thebelt to a transfer station C whereat a sheet of copy paper is moved at aspeed in synchronism with the moving belt in order to accomplishtransfer of the developed images. There is provided at this station asheet transport mechanismin the form of a transfer drum 20 adapted tosupport a sheet of paper and to carry the same into image transferrelationship with the belt 12 once for each image transfer operation. Asheet of paper S from a paper-handling mechanism, generally indicated bythe reference numeral 21, is transported into position upon the drum 20where it is supported during the image transfer function. The transferof the developed image from the selenium belt surface to sheet materialis effected by means of an electrical bias of the oppositepolarity asthe triboelectric charge on the developing particles utilized in imagedevelopment being applied to the transfer drum 20 at the point ofcontact between the sheet and selenium belt as the sheet passes thetransfer station C.

After the sheet is stripped from the transfer drum 20, it is conveyed byconveyor 22 into a fuser assembly generally indicatedby the referencenumeral 23 wherein the developed and transferred powder image on thesheet material is permanently affixed thereto. After fusing, thefinished copy is discharged from the apparatus at a suitable point forcollection externally of the apparatus.

It is believed that the foregoing description is sufficient for thepurposes of this application to show the general operation of anelectrostatic copier constructed in accordance with the presentinvention.

As shown in FIGS. 2 and 3, the unit 17 comprises a boxlike structure 35having a rectangular cross section and a length extending'beyond thewidth of the belt 12. Within the boxlike structure 35, there is suitablymounted a developer container formed with a thin walled developerhousing 36 closed at its ends, end walls 37 and 38. The housing 36contains developing material comprising carrier beads made frommagnetizable material and color toner particles which adhereelectrostatically in great numbers to the carrier beads. Mounted forrotation within the developer housing 36 are two magnetic brushes 39 and40 positioned with their axes in parallel and below the selenium belt12.

The magnetic brush 40, comprising outer cylinder 41 made ofnonmagnelizable material and extending almost the length of the housing36, is mounted for rotation within the structure 35. One end of thecylinder 41 is closed by a cap 42 which supports a drive shaft 43 inaxial alignment with the cylinder and is mounted in bearings 44 on theend plate wall 38. The other end of the cylinder is provided with a cap45 having a central opening therein. Within the cylinder 41 there ispositioned an elongated magnet bar 46 extending nearly the full lengthof the cylinder and being mounted therein by means of a stub shaft 47rotatably supported in the end cap 42 and a drive shaft 48 extendingthrough and rotatably mounted in bearings held within the centralopening formed in the cap 45. The shaft 48 extends through and issuitably joumaled in the end wall 37 so as to be rotated by an externalcontrol device as will be described hereinafter. In operation during adevelopment cycle, the brush cylinder 41 is rotated by way of the driveshaft 43 and the magnet 46 remains stationary.

The second magnetic brush 39 comprises a cylinder 50 having a length anddiameter equal to the cylinder 41. Within the brush cylinder 50 there ismounted a main magnet bar 51 which is supported in fixed positionrelative to its enclosing rotatable cylinder. This is accomplished bythe use of shafts (not shown) mounted at both ends of the magnet bar 51and projecting through openings in end caps (not shown) that are used toclose and support the ends of the cylinder 50. Such caps and shafts aresimilar to the cap 42 and the shaft 47 at one end and the cap 45 andshaft 48 at the other end. One of the shafts would be fixed in order tomaintain the magnet bar 51 in a fixed position during rotation of thecylinder 50. A second elongated magnet bar 52 is mounted within thecylinder 50 and is attached to a spacer bar 53 secured to the lowersurface of the magnet bar 51. The polar orientation of the magnets 46and 51 are indicated in FIG. 2 and are arranged so that magnetic linesof flux project through the walls of the respective cylinders 41, 50 andacross the surface of the belt 12 as it moves adjacent the brushes 39,40.

As shown in FIG. 2, the peripheral walls of the brush cylinders 41 and51 are relatively close to each other. During a development cycle whenboth cylinders are rotating in unison and with the magnets 46 and 51held stationary, the brush bristles produced by the sue of magnetizablecarrier beads in the developing material used in the unit 117 will formon the upper region of the cylinder 50 between this region and theundersurface of the selenium belt 12. These bristles remain formedduring the developing cycle, being produced by the magnetic field of themagnet 51 beginning slightly before reaching the closest distancebetween the cylinder 50 and the belt 12. When the bristles are moved outof the influence of this magnet beyond the closest distance between thebelt and the cylinder, they are picked up by the magnetic field of themagnet 46 which is stronger at this point than the diminished strengthof he magnetic field attributed to the magnet 51 and carried therealongduring rotation of the cylinder 41 until they reach a point beyond thedevelopment zone Z when the carrier beads and toner particles drop offthe cylinder and back into the housing 36.

During movement of the carrier beads and toner through the developmentzone Z, the magnetic bristles and, therefore, the development material,is in the form of a magnetic blanket" extending the entire width of thezone Z wherein the material is disposed or available for developingpurposes. It will be apparent that the width of the development zone Zis larger than the sum of the individual development zones for each ofthe magnetic brushes 39, 40. One or two additional brushes may be addedin the same arrangement in order to extend the dimensions of themagnetic blanket and therefore Also mounted within the developmenthousing 36 and below the magnetic brush 39 is an impeller 54 having aplurality of blades 55 radially extending therefrom and having one endrotatably mounted in the end wall 37 and its other end terminating in adrive shaft 46 which in turn is rotatably mounted on and extends throughthe end wall 38. During a development cycle the impeller 54 is rotatedin the direction shown by the arrow in FIG. 2 and serves to carry andthrow development material toward the lower surface of the magneticbrush 39. The development material so thrown is picked up by the pickupmagnet 52 which commences the formation of bristles on the cylinder 50.As this cylinder rotates, the newly formed bristles come under theinfluence of the main magnet 51.

A clipping blade 57 is secured to the upper wall of the housing 36 andextends radially toward the cylinder 41 being spaced from the peripherythereof a short distance equal to the length of the bristles to beformed on this magnetic brush. Upon rotation of the cylinder 50 forcarrying the magnetic bristles toward the development zone Z, the blade57 clips the tops of the magnetic bristles for presenting optimumlengths of the bristles to the belt 12. A curved baffle plate 58 issecured interiorly of the housing 36 and extends along for a lengthequal to the length of the impeller 54 with the center of curvaturecoincident with that of the impeller. The baffle assists the paddleblades of the impeller in forming large increments of developingmaterial to be moved from the lower region of the housing 36 to thevicinity of the pickup magnet 52 whereat some of the material is formedinto magnetic brush bristles.

A pair of augers 60, 61 are mounted in the lower region of the housing36 for insuring continual mixing of the particles that comprise thedeveloped material and for insuring proper quantities at all times. Eachof the augers is suitably mounted at one end on the wall 37 while theother ends terminate in a drive shaft 62 which extends through andsupported on the end wall 38. A suitable drive mechanism (not shown) maybe connected to the shaft 62 and be activated when the machine is in itsoperative condition.

The magnetic developer unit 17 is also provided with devices which willcontrol the developing action of the unit, and in such a way that theaction may be switched on and off as rapidly as possible. Since thedevelopment zone Z is relatively wide and more than one magnetic brushis being controlled, the time period in which it takes the magneticblanket" over both brushes to become inoperative is shortened by havingboth brushes controlled separately. To this end, the first of themagnetic brushes; namely, the brush 39 is provided with a gate element64 fixed to a shaft 65 for rotation therewith. The gate 64 is formedwith an edge 66 extending radially relative to the cylinder 50, and uponthe rotation of the shaft 65, is adapted to engage the adjacentperiphery of this cylinder. This action serves to scrape off instantlyany magnetic bristles on the cylinder 50 thereby preventing thetransport of development material beyond the edge 66 and consequentlyterminating the development action by the magnetic brush 39 except forthe developing material still on the brush the instant before gating.This control action of the shaft 65 is made effective by the overallmachine-programming system.

Gating action of the other magnetic brush 40 is provided by the quickrotation of the magnet 46 for approximately from its illustratedposition. As previously stated, the magnet 46 is provided with a shaft48 that extends externally of the developer housing 36. As shovm in FIG.2, the shaft 48 is rotatably connected to a rocker arm 67 which has apin 68 extending therefrom at one end to be engaged by a rocker arm 69arranged to be swung in either direction upon activation of a rotarysolenoid SOL-l. This solenoid is suitably mounted on the end wall 37 andconnected electrically to the programming system for the machine. Uponenergization of the solenoid SOL-l when it is desired to inactivate theunit 17, the rocker arm 69 is rocked in one direction to producingrocking of the rocker arm 67 with corresponding rotation of the shaft47. This complete action involves only a very small increment of time.Rotation of the magnet 46 will remove its magnetic influence upon themagnetic carrier beads attempting to bridge across the gap between themagnetic brushes 39, 40. A suitable machine-programming system may bearranged to efi'ect simultaneously activity of the shaft 65 and theshaft 48 for causing near simultaneous gating of the two magneticbrushes. In this event, only that developing material which forms on thebrush 39 beyond the edge 66 will be involved in further developmentbefore most of the material falls through the space between thecylinders 41, 50 and, for the brush 40, that development material whichwas conveyed from the magnetic brush 39 just prior to gating requirementwill be involved in further development. In this manner, the length oftime needed to remove all residual developing material from the magneticblanket on and between the brushes 39 and 40 when gating is programmedis effectively that time needed to remove the material from just one ofthe brushes. As opposed to the use of a magnetic conveyor belt betweentwo rollers, an arrangement which cannot be effectively gated to on-offconditions in acceptable short periods of time, the present magneticblanket" concept, especially with the employment of many individualmagnetic brushes, has the advantage that the gating periods for theentire coverage of the blanket is effectively the same for only one ofthe brushes utilized.

While the invention has been described with reference to the structuredisclosed herein, it is not confined to the details set forth; but isintended to cover such modifications, or changes as may come within thescope of the following claims.

What is claimed is:

1. An apparatus for developing a movable surface bearing a latentelectrostatic charge image including:

a plurality of rotatable cylindrical members mounted in spaced relationwith an open space therebetween, said members having their axesgenerally in parallel and being arranged in spaced tangentialrelationship to the surface,

each of said members having associated therewith means for producing amagnetic field across a portion of the periphery thereof and across saidsurface as the same moves, said last-named means being arranged toproduce bristles of magnetizable particles on each of the members duringrotation thereof,

means for supplying a mixture of developing material having magnetizableparticles and developing powder electrostatically carried by theparticles to at least one of the members thereby initiating theformation of bristles thereon,

said means for producing magnetic fields being arranged wherein themagnetic influence upon one member diminishes within a stronger magneticinfluence of the other thereby causing the movement of bristles fromsaid one member to said other member across said open space therebetweenupon rotation of said members.

2. An apparatus for developing a latent electrostatic image on a movingphotoconductive insulating surface including:

a housing for containing a supply of developing material comprisingmagnetizable carrier particles and electrostatically adhering developingparticles,

a plurality of cylindrical members mounted in spaced relation in thehousing with an open space therebetween and arranged adjacent to and inspaced tangential relationship with said photoconductive insulatingsurface with their longitudinal axes generally normal to the path ofmovement of said surface,

means for rotating said members,

means supported within the cylindrical members for producing a magneticfield across at least a portion of the periphery of each of the members,and wherein the magnetic fields produced overlie one another wherebyupon rotation thereof causing the formation of bristles of developingmaterial upon said portion of each member arranged to contact saidsurface during movement thereof,

said means for producing a magnetic field being cooperable with saidrotating members for producing movement of bristles along said portionof one of the members across said open space between the same and anadjacent member and along said portion of the adjacent member with thedeveloping material remaining in contact with said surface therebypresenting a partially unsupported magnetic developing blanket duringdevelopment of the latent electrostatic image.

3. An apparatus for developing a movable surface bearing a latentelectrostatic charge image including: 7

a plurality of rotatable cylindrical members mounted in spaced relationwith an open space therebetween, said members having their axesgenerally in parallel and being arranged in spaced tangentialrelationship to the surface,

each of said members having supported therein means for producing amagnetic field across a portion of the periphery thereof and across saidsurface as the same moves, said last named means being arranged toproduce bristles of magnetizable particles on each of the members duringrotation thereof,

at least one of said members being positioned to receive a supply ofdeveloping material having magnetizable particles and developing powderelectrostatically carried by the particles thereby initiating theformation of bristles thereon,

said means for producing magnetic fields being arranged wherein themagneticv influence upon one member diminishes within a strongermagnetic influence of the other thereby causing the movement of bristlesfrom said one member to said other member across said open spacetherebetween upon rotation of said members.

4. An apparatus for developing a movable surface bearing a latentelectrostatic charge image including:

a plurality of rotatable elongated members mounted in spaced relationwith an open space therebetween, said members having their longitudinaldimension generally in parallel and being arranged in spacedrelationship to the surface,

each of said members having associated therewith means for producing amagnetic field over a portion of and along said surface as the samemoves, said last-named means being arranged to produce bristles ofmagnetizable particles on each of the members,

means for supplying a mixture of developing material having magnetizableparticles and developing powder electrostatically carried by theparticles to at least one of the members thereby initiating theformation of bristles thereon,

said means for producing magnetic fields being arranged wherein themagnetic influence upon one member diminishes within a stronger magneticinfluence of the other thereby causing the movement of bristles fromsaid one member to said other member across said open space therebetweenupon rotation of said members.

5. An apparatus for developing a latent electrostatic image on a movingphotoconductive insulating surface including:

a housing for containing a supply of developing material comprisingmagnetizable carrier particles and electrostatically adhering developingparticles,

a plurality of rotatable elongated members mounted in spaced relation inthe housing an open space therebetween and arranged adjacent to and inspaced relationship with said photoconductive insulating surface withtheir longitudinal axes generally normal to the path of movement of saidsurface,

means supported within the members for producing a magnetic field overat least a portion of each of the members and causing the formation ofbristles of developing material upon said portion of each member thebristles being arranged to contact said surface during movement thereof,said members being disposed in close parallel relationship whereby themagnetic fields associated with said members overlap,

said means for producing a magnetic field being cooperable with saidmembers upon rotation of said members for producing movement of bristlesalong said portion of one of the members across said open space betweenthe same and an adjacent member and along said portion of the adjacentmember with the developing material remaining in contact with saidsurface thereby presenting a partially unsupported magnetic developingblanket during development of the latent electrostatic image.

6. In an electrostatic reproduction machine having means for producinglatent electrostatic images on an element supporting latent images andfor processing the images into copies of an original including:

a first magnetic developing brush arranged for contacting and applyingdeveloping material upon the images being processed,

a second magnetic developing brush arranged for contacting and applyingdeveloper material upon the images after the same have been contacted bysaid first brush,

said first and second magnetic brushes comprising a pair of rotatableroll members having an open space therebetween and means to establish amagnetic field over at least a portion of each member,

the magnetic fields associated with said members overlapping across saidopen space,

said second brush exerting a stronger magnetic influence, at a point insaid open space, than said first brush, for

receiving developing material therefrom.

whereby said second brush is arranged to receive its supply ofdeveloping material from said first brush across the space therebetweenupon rotation of said members.

7. In an electrostatic reproduction machine having means for producinglatent electrostatic images on an element supporting latent images andfor processing the images into copies of an original including:

a first magnetic developing brush arranged for contacting and applyingmagnetizable developing material upon the images being processed,

a second magnetic developing brush arranged for contacting and applyingdeveloper material upon the images after the same have been contacted bysaid first brush,

said first and second magnetic brushes comprising a pair of rotatableroll members having an open space therebetween and means to establish amagnetic field over at least a portion of each member,

the magnetic fields associated with said members overlapping across saidopen space,

said second brush exerting a stronger magnetic influence, at a point insaid open space, than said first brush, for receiving developingmaterial therefrom.

whereby the magnetizable material is transported across said open spacefrom one brush to another upon rotation of said members.

1. An apparatus for developing a movable surface bearing a latentelectrostatic charge image including: a plurality of rotatablecylindrical members mounted in spaced relation with an open spacetherebetween, said membeRs having their axes generally in parallel andbeing arranged in spaced tangential relationship to the surface, each ofsaid members having associated therewith means for producing a magneticfield across a portion of the periphery thereof and across said surfaceas the same moves, said lastnamed means being arranged to producebristles of magnetizable particles on each of the members duringrotation thereof, means for supplying a mixture of developing materialhaving magnetizable particles and developing powder electrostaticallycarried by the particles to at least one of the members therebyinitiating the formation of bristles thereon, said means for producingmagnetic fields being arranged wherein the magnetic influence upon onemember diminishes within a stronger magnetic influence of the otherthereby causing the movement of bristles from said one member to saidother member across said open space therebetween upon rotation of saidmembers.
 2. An apparatus for developing a latent electrostatic image ona moving photoconductive insulating surface including: a housing forcontaining a supply of developing material comprising magnetizablecarrier particles and electrostatically adhering developing particles, aplurality of cylindrical members mounted in spaced relation in thehousing with an open space therebetween and arranged adjacent to and inspaced tangential relationship with said photoconductive insulatingsurface with their longitudinal axes generally normal to the path ofmovement of said surface, means for rotating said members, meanssupported within the cylindrical members for producing a magnetic fieldacross at least a portion of the periphery of each of the members, andwherein the magnetic fields produced overlie one another whereby uponrotation thereof causing the formation of bristles of developingmaterial upon said portion of each member arranged to contact saidsurface during movement thereof, said means for producing a magneticfield being cooperable with said rotating members for producing movementof bristles along said portion of one of the members across said openspace between the same and an adjacent member and along said portion ofthe adjacent member with the developing material remaining in contactwith said surface thereby presenting a partially unsupported magneticdeveloping blanket during development of the latent electrostatic image.3. An apparatus for developing a movable surface bearing a latentelectrostatic charge image including: a plurality of rotatablecylindrical members mounted in spaced relation with an open spacetherebetween, said members having their axes generally in parallel andbeing arranged in spaced tangential relationship to the surface, each ofsaid members having supported therein means for producing a magneticfield across a portion of the periphery thereof and across said surfaceas the same moves, said last named means being arranged to producebristles of magnetizable particles on each of the members duringrotation thereof, at least one of said members being positioned toreceive a supply of developing material having magnetizable particlesand developing powder electrostatically carried by the particles therebyinitiating the formation of bristles thereon, said means for producingmagnetic fields being arranged wherein the magnetic influence upon onemember diminishes within a stronger magnetic influence of the otherthereby causing the movement of bristles from said one member to saidother member across said open space therebetween upon rotation of saidmembers.
 4. An apparatus for developing a movable surface bearing alatent electrostatic charge image including: a plurality of rotatableelongated members mounted in spaced relation with an open spacetherebetween, said members having their longitudinal dimension generallyin parallel and being arranged in spaced relationship to the surface,each of said members having associatEd therewith means for producing amagnetic field over a portion of and along said surface as the samemoves, said last-named means being arranged to produce bristles ofmagnetizable particles on each of the members, means for supplying amixture of developing material having magnetizable particles anddeveloping powder electrostatically carried by the particles to at leastone of the members thereby initiating the formation of bristles thereon,said means for producing magnetic fields being arranged wherein themagnetic influence upon one member diminishes within a stronger magneticinfluence of the other thereby causing the movement of bristles fromsaid one member to said other member across said open space therebetweenupon rotation of said members.
 5. An apparatus for developing a latentelectrostatic image on a moving photoconductive insulating surfaceincluding: a housing for containing a supply of developing materialcomprising magnetizable carrier particles and electrostatically adheringdeveloping particles, a plurality of rotatable elongated members mountedin spaced relation in the housing with an open space therebetween andarranged adjacent to and in spaced relationship with saidphotoconductive insulating surface with their longitudinal axesgenerally normal to the path of movement of said surface, meanssupported within the members for producing a magnetic field over atleast a portion of each of the members and causing the formation ofbristles of developing material upon said portion of each member thebristles being arranged to contact said surface during movement thereof,said members being disposed in close parallel relationship whereby themagnetic fields associated with said members overlap, said means forproducing a magnetic field being cooperable with said members uponrotation of said members for producing movement of bristles along saidportion of one of the members across said open space between the sameand an adjacent member and along said portion of the adjacent memberwith the developing material remaining in contact with said surfacethereby presenting a partially unsupported magnetic developing blanketduring development of the latent electrostatic image.
 6. In anelectrostatic reproduction machine having means for producing latentelectrostatic images on an element supporting latent images and forprocessing the images into copies of an original including: a firstmagnetic developing brush arranged for contacting and applyingdeveloping material upon the images being processed, a second magneticdeveloping brush arranged for contacting and applying developer materialupon the images after the same have been contacted by said first brush,said first and second magnetic brushes comprising a pair of rotatableroll members having an open space therebetween and means to establish amagnetic field over at least a portion of each member, the magneticfields associated with said members overlapping across said open space,said second brush exerting a stronger magnetic influence, at a point insaid open space, than said first brush, for receiving developingmaterial therefrom. whereby said second brush is arranged to receive itssupply of developing material from said first brush across the spacetherebetween upon rotation of said members.
 7. In an electrostaticreproduction machine having means for producing latent electrostaticimages on an element supporting latent images and for processing theimages into copies of an original including: a first magnetic developingbrush arranged for contacting and applying magnetizable developingmaterial upon the images being processed, a second magnetic developingbrush arranged for contacting and applying developer material upon theimages after the same have been contacted by said first brush, saidfirst and second magnetic brushes comprising a pair of rotatable rollmembers having an open space therebetween and means to esTablish amagnetic field over at least a portion of each member, the magneticfields associated with said members overlapping across said open space,said second brush exerting a stronger magnetic influence, at a point insaid open space, than said first brush, for receiving developingmaterial therefrom. whereby the magnetizable material is transportedacross said open space from one brush to another upon rotation of saidmembers.